[poonilization]

So here's the question



I've answered parts a and b.
For part b, both isomers lowest energy conformer is when the ^tBu group is in an equatorial position to reduce unfavourable 1,3 diaxial interactions. But i'm not sure why the cis isomer proceeds with a faster rate of oxidation than the trans isomer. Any ideas?


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[AromaticAcrobatic]

Which isomer is more stable (less reactive) and why?
Answering this question should guide you to the correct answer.
 

[poonilization]

I think the trans isomer would be more stable (less reactive) because the OH group is equatorial whereas the cis isomer the OH group is axial and therefore is subject to unfavourable 1,3 diaxial interactions.

So the question states the cis isomer reacts faster, i would guess this has something to do with losing the axial OH group and forming a ketone which would be more energetically favourable?

[AromaticAcrobatic]

I think the trans isomer would be more stable (less reactive) because the OH group is equatorial whereas the cis isomer the OH group is axial and therefore is subject to unfavourable 1,3 diaxial interactions.

Very good!

The second part is basically correct. Losing an axial OH would decrease the energy of the molecule, but is one isomer more accessible then another? Would this have something to with how fast the reaction occurs?

[poonilization]

I'm not too sure, in regards to the sterics, both the trans and cis isomers aren't directly adjacent to any bulky groups so i don't see which isomer's OH group would be more accessible. I would have thought that because the cis isomer has the OH group axial there would be a very small amount of steric clash, so it would be harder for the larger chromic acid to attack from the axial position therefore wouldn't the trans isomer have a faster rate of reaction? But the question states that the cis isomer has a faster rate of reaction which is why i'm confused.

Or slightly reversing what i just said, is it because of the fact that the cis isomer has the OH group in the axial position, the energy of the molecule is higher than the trans isomer, and therefore there is a greater thermodynamic driving force and larger ΔE for the cis reaction than trans reaction.

However i'm still unsure about the answer to your question, which isomer is more accessible than the other?

With regards to the second part of the question, would the rate determining step be when the lone pair on the oxygen of the cis isomer attacks chromic acid as this forms a bulky group that is axial and clashes with neighbouring axial H atoms (uphill on the reaction energy profile diagram), and then the fast step would be either the expulsion of the chromic acid or water to form a double bond (downhill on the diagram).

[AromaticAcrobatic]

With regards to the second part of the question, would the rate determining step be when the lone pair on the oxygen of the cis isomer attacks chromic acid as this forms a bulky group that is axial and clashes with neighbouring axial H atoms (uphill on the reaction energy profile diagram), and then the fast step would be either the expulsion of the chromic acid or water to form a double bond (downhill on the diagram).

I think the rate determining step would be the initial proton transfer, as it seems once this happens then chromic acid has a good leaving on it and is more electrophilic because of the leaving group, which then you generate the H2O needed to deprotanate the OH alpha Hydrogen which creates the Ketone.  I.e once the initial proton transfer step happens the reaction spirals to completion.

Or slightly reversing what i just said, is it because of the fact that the cis isomer has the OH group in the axial position, the energy of the molecule is higher than the trans isomer, and therefore there is a greater thermodynamic driving force and larger ΔE for the cis reaction than trans reaction.

^ That is pretty much it. There is a greater thermodynamic driving force for the cis reaction then the trans reaction. This is because with the cis isomer the OH group is more accessible by it swinging around bumping into its neighbors which in turn makes it move more which means it bumps into chromic acid molecules more which allows the initial rate determining step to occur quicker then the trans isomer at which the OH group is just hanging out, kind of protected by its neighbors.

 

[Altered State]

Or slightly reversing what i just said, is it because of the fact that the cis isomer has the OH group in the axial position, the energy of the molecule is higher than the trans isomer, and therefore there is a greater thermodynamic driving force and larger ΔE for the cis reaction than trans reaction.

^ That is pretty much it. There is a greater thermodynamic driving force for the cis reaction then the trans reaction. This is because with the cis isomer the OH group is more accessible by it swinging around bumping into its neighbors which in turn makes it move more which means it bumps into chromic acid molecules more which allows the initial rate determining step to occur quicker then the trans isomer at which the OH group is just hanging out, kind of protected by its neighbors.

 

Hum, I totally agree with the thermodynamic point of view, and it's derived driving force, but I do not see the kinetic explanation so straightforward...
I mean, you are probably right, and the probability of an axial  OH to *Ignore me, I am impatient* into a chromium oxide is bigger than the one for an ecuatorial OH, but it seems to me counter-intuitive: if I had to choose one of the two to be more available, I would say the equatorial since is like "more away" of the rest of the chair, and the axial one, ",ore directed towards the center" and kinda "more trapped". But in any way, I this is totally not straightforward, and we would need some calculations (maybe done somewhere I did not look?) to state that...

I understand the justification as it follows:
The cis isomer is higher in energy than the trans isomer, and both activated transition states have similar energies, then the cis isomer will have less activation energy than the trans one therefore faster reaction.

But I am still not sure, since the intermediates formed consist on the same molecule but with a bulkier sustitutent, and that is supposed to raise the energy of the one with an axial -O(Cr(=O)3).


I'm not trying to confuse the OP, but I never thought on this situation before, can't be sure that I can properly explain it. Hope someone can enlighten us

[poonilization]

Thanks for the reply and i agree, i was confused because i thought OH on the trans isomer was more accessible as its equatorial and to the side of the molecule even though the question states the cis isomer reacts faster.

I think the OH on the cis isomer is supposed to be more accessible as it is perpendicular to the plane of the molecule but im not entirely sure.

[Altered State]

Lol, I've just realized that the forum censured my "*bu-mp" word into an impatience statement message :-D

[Dan]

To answer this question you need to think about the intermediates in the reaction mechanism and consider where the rate limiting step might be. Since the reaction is irreversible under the reaction conditions, a kinetic argument is required.

Breaking it down, we have two main steps:

1. Formation of the chromate ester
2. Breakdown of the chromate ester (resulting in the ketone an Cr(IV))

The discussion so far seems to have focussed only on 1. - for this step we expect the less hindered OH to react faster. There seems to be some disagreement over which one this is. The equatorial OH is less hindered than the axial (1,3-transannular strain). So it would be reasonable to expect that the trans isomer will form a chromate ester more easily than the cis, because build up of strain in the transition state is greater for the axial OH than the equatorial. The apparently contradictory experimental observation that the cis reacts faster may imply that step 1. is not rate limiting...

What about step 2.? Will the axial or equatorial chromate ester break down faster, and why?